1. Field of the Invention
This invention relates to heated offshore pipelines, more particularly to pipelines electrically heated by AC current flowing through a pipeline made of electrically conducting pipe that is electrically insulated from the sea water with a waterproof coating.
2. Related Art
The use of the technique known as "subsea", or "offset" production to produce offshore oil and gas from offshore reservoirs is increasing. This technique employs small, submerged pipelines known as "flow lines" that are laid along relatively short routes between the submerged wellheads and a gathering point such as a nearby platform. For a variety of reasons it will often be beneficial or necessary to take measures to maintain high temperatures in the fluid flowing in subsea flow lines, One way to this is done is to add insulation.
Because flow lines are generally small, relatively short, and often in deep water, installation from reel barges is often faster and less expensive than installation using lay barges which depend on stringing the pipe joints together on deck. Reeling, however, generally precludes the use of a concentric pipe with an insulation-filled annulus except on very small flow lines, due to the stiffness of the composite pipe. Various plastic, rubber or foamed plastic coatings have been used to insulate reel laid pipes, but these are both more expensive for a given volume, and more conductive than conventional insulating materials that can be used in a concentric pipe design.
Because several flow lines are often laid along the same short route, it is sometimes less expensive to assemble the flow lines onshore and tow a bundle of several pipelines to the offshore site, particularly if insulation is needed. These bundles are usually assembled inside a "carrier" pipe that serves to reduce submerged weight and to protect the flow lines during the journey. The bundle may be towed off-bottom at a "controlled-depth", in which case the weight of the bundle is accurately controlled to be very near to the buoyant force, or it may be kept just slightly heavier than the buoyant force so that it stays on the bottom during trip without too much drag. In the controlled depth tow method, the void space between the carrier pipe and the flow line bundle is generally filled with ballast to sink the pipeline bundle when it reaches its destination. Liquids or slurries that partially solidify have been used to serve the combined purpose of insulation and ballast. For bundles that are towed along the sea bed, the additional ballast provided by filling the flow lines themselves may be adequate. In such cases, inexpensive, conventional insulating materials have been installed in the dry space between the bundled flow lines and the casing pipe. These insulating materials are more effective and less expensive than the submersible insulating materials used on reeled pipelines. This is taken to be a considerable advantage of this method of installing flow line bundles when insulation is desirable or necessary. In some cases this can be the deciding factor in whether to tow the pipelines, or lay them separately from a reel barge. One disadvantage of housing the bundled lines inside a casing pipe is that in very deep water the wall thickness required to keep the casing pipe from collapsing under the hydrostatic pressure makes it difficult to achieve the desired buoyancy. This has been overcome in some cases by pressurizing the casing pipe with nitrogen, but pressurizing also adds expense due to the cost of the nitrogen itself, the required filling apparatus and time to fill the casing pipe. The flotation pipe in this method is also larger than would be required to add the required buoyancy, because part of the volume of the casing pipe is occupied by the flow lines. This is space that does not contribute buoyant force as it would if the flow lines were outside the floatation pipe. Another disadvantage of this design is that some sort of carriage system is needed to get the flow line bundle pulled into the casing pipe. It would be quite difficult to connect one prefabricated length of such a bundle to another that is already in the water. Precisely because of this difficulty, most towed pipeline bundles have been fabricated in a single section on shore. This requires a very long section of land with a beach front.
The benefit of increasing the thickness of a given insulating material diminishes with its thickness, but the benefit of decreasing the conductivity of a given thickness of insulating material increases linearly, and inversely. It is, therefore, not possible to generalize when comparing the merits of a low conductivity material applied one way to a high conductivity material applied in a different way. For long pipelines it may be impractical to add enough insulation to achieve the desired result. In other cases it may be practical to adequately insulate with a low conductivity material, but not a higher conductivity material. In either case, the practical length can be increased by adding more heat than is contained in the produced fluids. Heating can also be useful and cost-effective even where it is not essential to operating the pipeline.
Heating pipelines is a common practice on land, but only a few offshore pipelines have been heated. One reason for heating pipelines is that many of the problems of cooling in pipelines are of a temporary or transient nature due to changing flow conditions. Some insulated pipelines carry materials that will freeze or turn to wax if flow is stopped, but which at full flow can be transported through the pipe fast enough to avoid cooling problems. In such cases, the pipeline must quickly be flushed with a different fluid after it is shut down. Insulation can be used to slow heat loss enough to allow time to flush the line in an un-planned shut down but the ability to heat the pipeline can provide an infinite "shut-down window" and eliminate the need to flush the line. Another type of transient flow condition is declining flow rates that occur in flow lines that carry subsea well production. Because temperature drop depends on the length of time the fluid is in the pipe, the temperature loss increases as the flow rate decreases. In such cases the peak power need for heating may not occur until near the end of the life of the oil or gas well, meaning that much of the cost of heating is deferred. If a less effective insulating system can be installed at lower cost, then the combined cost of heat and insulation may be lower than the cost of a more effective insulation system. The problems associated with temperature are not entirely predictable, so with non-heated line the worst case must be paid for in advance. The heated line can be operated with just enough heat to suit the real need.
Pipelines can be heated by pumping hot water or steam through a separate heating pipes that are thermally connected to the pipeline. Where waste heat is not available, electric heating usually more cost effective, but electric heating has rarely been used offshore due to the complications of using high power heating underwater.
One of the oldest ways to electrically heat pipelines is to use the pipe itself as an impedance heating element. For pipes made of magnetic material, alternating current tends to flow near the outside surface of the pipe due to self-induced eddy currents in the pipe wall. If the pipe wall is thick, this can effectively insulate the current from the fluid inside the pipe, thus allowing this method to be used even when the fluid flowing in the pipeline is somewhat conductive. While conceptually very simple, this method has several disadvantages, depending on the case. Because the resistance of steel pipelines is low, high currents are needed to generate enough heat to significantly effect the fluid temperature. Considerable power is lost through the return path unless resistance is quite low. This requires large return cables, and generally precludes earth as the return path. Furthermore, insulating materials typically used to insulate land pipelines are not designed as electrical insulators, and are not suitable for insulating high voltage. Building codes, therefore, limit the voltages to levels that are not hazardous to humans. This effectively limits the use of this heating method to pipelines shorter than those typically used offshore.
One method that has been used to overcome these problems is to cause induction heating in one or more small pipes that are attached to the pipeline and house an electrical conductor that carries high voltage alternating current, using the small pipe as the return current path. Heat is induced in the small pipe, but the same "skin effect" phenomenon that causes current to flow on the outside of a single conductor causes the current to flow on the inside of the magnetic outer conductor of coaxial conductors carrying current in opposite directions. This method, therefore, insulates both the outside and the inside of the pipeline, allowing high voltage heating of conductive fluids without risk of electrical shock. This is described in U.S. Pat. Nos. 3,617,699, 3,777,117 and 3,975,617 and is known as skin effect heat tracing. It is common on land, and it has been used on at least one offshore pipeline. In that pipeline, the heating tubes were pre-installed on individual pipe joints, over which larger, concentric pipes were installed from the end to house the foam insulation. Each pipe joint contained a junction box that allowed connection of the heating tubes and wires offshore, after two pipes were joined. Split sleeves were installed over the joint, and welded. Because the pipes were welded offshore during the lay process, this a time consuming process was very expensive.
U.S. Pat. No. 5,241,147 addresses these problems by proposing to induce heat in the pipe with a magnetic field created by passing an AC current through wires placed outside the insulation. This, however, may leave the wires vulnerable.
U.S. Pat. No. 3,975,617 discloses that conductors placed near the pipe can be used for the combined purpose of inducing heat in the pipe and power transmission to production equipment.
One method previously noted teaches that the steel windings in flexible pipe of the type used offshore can be used as a direct current, resistive heating element with earth as the return current path. This depends plastic or rubber layers in hose or flexible pipe electrically insulating the steel windings from sea water, and from conductive fluids inside the pipeline. The disadvantage of using DC current is that the sea water will act as an electrolyte, and the steel windings could be rapidly consumed by electrolysis.